Liquid crystal display devices are in wide use as thin and light flat displays for use in various electronic machines. There are several display schemes used in liquid crystal display devices. Among those, a scheme known as IPS (In-Plane Switching), in which an electric field is applied to liquid crystal in parallel to a substrate for obtaining a wide viewing angle, is suitably used for monitor displays for use in personal computers, liquid crystal TV sets or the like because of its excellent image properties.
Liquid crystal display devices using IPS are disclosed in Japanese Unexamined Patent Publication No. 10-10556, for example. A plan view of a pixel portion thereof is shown in FIG. 47. Such a liquid crystal display device comprises an array substrate and an opposing substrate parallel to each other, and liquid crystal held between the array substrate and the opposing substrate. As shown in FIG. 47, in the array substrate, gate wirings 101 feeding scanning signals and source wirings 102 feeding image signals are arranged so as to intersect at approximately right angles. Nearby each intersection of the gate wiring 101 and the source wiring 102, a thin-film transistor (TFT) 104 having a semiconductor layer is formed as a switching element. To the source wiring 102, a comb-like pixel electrode 115 is connected via the TFT 104. Opposing electrodes 116 functioning as a standard potential are arranged so as to mesh with the pixel electrode 115. The opposing electrodes 116 are electrically connected to a common wiring 103 parallel to the gate wiring 101 through a contact hole 108. At the intersection of the common wiring 103 and the pixel electrode 115, with an insulating layer (not shown) in between, a storage capacitor region 107 is formed.
According to such a liquid crystal display device, an electric field substantially parallel to the substrates is generated by the difference between the voltage applied to the pixel electrode 115 and that of the opposing electrode 116, to which a standard potential is applied, and thereby the liquid crystal (not shown) held between the electrodes is driven. By storing electric charge in the storage capacitor region 107 while the TFT 104 is in an on-status, the liquid crystal remains actuated while the TFT 104 is in an off-status.
In prior art IPS style liquid crystal display devices, pixel electrodes and opposing electrodes are generally made of aluminum or the like metals. Therefore, the pixel electrodes and opposing electrodes do not transmit light, leading to the drawback of an unsatisfactory pixel aperture ratio. Japanese Unexamined Patent Publication No. 10-10556 proposes a way to enhance the aperture ratio by forming either or both of the pixel electrode 115 and the opposing electrode 116 out of a transparent conductive film.
In the case where both the pixel electrode 115 and the opposing electrode 116 are made of transparent electrodes, it is preferable that both the electrodes be formed as a same layer in order to avoid a more complicated production process and increased manufacturing costs. However, this arrangement may lower the manufacturing yield by causing short-circuits between the pixel electrode 115 and the opposing electrode 116. Therefore, it is more practical that either the pixel electrode or the opposing electrode be made of a transparent electrode.
However, forming only one of the pixel electrode and the opposing electrode out of a transparent electrode and forming the other out of metal or a like material may cause flicker due to the difference in the optical properties of the two materials.
In order to apply a sufficient voltage to liquid crystal molecules while preventing decomposition or deterioration thereof, liquid crystal display devices are driven by the alternating current drive method, where an electric potential alternately positive and negative relative to that of the opposing electrode is applied to the pixel electrode at a regular interval (for example, once every sixtieth seconds). When the alternating current drive method is employed in a liquid crystal display device in which only one of the pixel electrode and the opposing electrode is a transparent electrode, its transmittance changes cyclically between the period when an electric potential positive relative to that of the opposing electrode (positive frame) is applied to the pixel electrode and the period when an electric potential negative relative to that of the opposing electrode (negative frame) is applied to the pixel electrode, causing observable differences in brightness.